The invention relates to the formation a coating on the surface of a substrate, for example, for eventual formation of optical devices or electrical devices. In particular, the invention relates to highly uniform particle coatings on substrates and to efficient ways of forming highly uniform particle coatings that can be further processed to form glasses and other highly uniform coatings on a substrate.
The consolidation or integration of mechanical, electrical and optical components into integral devices has created enormous demands on material processing. Furthermore, the individual components integrated in the devices are shrinking in size. Therefore, there is considerable interest in the formation of specific compositions applied to substrates. In order to form optical devices with high quality optical coatings from these materials, the coatings must be highly uniform. Interest in forming highly uniform materials for these coatings has sparked the development of processes.
Presently used optical communication light wavelengths are from 1.3 to 1.6 microns. Optical waveguides generally have dimensions many times the wavelength. Thus, optical structures can have dimensions from a few microns to about 100 microns depending on optical mode requirements and other factors.
An explosion of communication and information technologies including internet based systems has motivated a world wide effort to implement fiber optical communication networks to take advantage of a very large bandwidth. The capacity of optical fiber technology can be expanded further with implementation of Dense Wavelength Division Multiplexing technology. With increasing demands more channels are needed to fulfill the system functions. Integrated components can be used to replace discrete optical components to supply the desired capacity.
Optical components can be integrated onto a planar chip-type base similar to an electronic integrated circuit. By placing the optical components onto an integrated chip such as a silicon wafer, many optical components can be squeezed into a very small foot print. For the mass production of these integrated optical chips, existing semiconductor technology, such as lithography and dry etching, can be involved advantageously in appropriate steps of the production process.
The production of integrated optical components requires the deposition of high quality optical materials onto the substrate surface. Furthermore, the optical materials must be fashioned into specific devices. In particular, a promising technology for the integration of optical components centers around the production of planar waveguides. Semiconductor approaches have been used to form the waveguides following the deposition of optical materials.
Basic characteristics of optical film coatings include surface quality, film uniformity and optical purity. Optical quality refers to small enough absorption and scattering loss to achieve desired levels of transmission. Optical quality also includes the uniformity of optical properties, such as index of refraction and bi-refringence properties. In addition, optical quality includes interface quality, such as the interface between the core layers and cladding layers. Current benchmarks are established, for example, by glass fibers, planar waveguide glass, lithium niobate, and InP. For silica (SiO2) preferred forms are a glass, while for other materials single crystal forms have the highest quality optical transmission.
Several approaches have been used and/or suggested for the deposition of the optical materials. These approaches include, for example, flame hydrolysis deposition, chemical vapor deposition, physical vapor deposition, sol-gel chemical deposition and ion implantation. Flame hydrolysis deposition has become the leader for commercial implementation of planar waveguides. Flame hydrolysis and forms of chemical vapor deposition have also been successful in the production of glass fibers for use as fiber optic elements. Flame hydrolysis deposition involves the use of a hydrogen-oxygen flame to react gaseous precursors to form particles of the optical material as a coating on the surface of the substrate. Subsequent heat treatment of the coating can result in the formation of a uniform optical material, which generally is a glass material.
No clear approach has been established as the leading contender for production of the next generation of integrated optical components that will have stricter tolerances for uniformity and purity. Flame hydrolysis deposition is efficient, but cannot be easily adapted to obtain more uniform coatings. Chemical vapor deposition involves the deposition of radicals, molecules and/or atoms onto the substrate surface rather than particles. Chemical vapor deposition can achieve very uniform materials, but the process is extremely slow. If attempts are made to increase the rates, the film quality is compromised, which reduces any advantage of the chemical vapor deposition process.
At the same time, approaches have been developed for the production of highly uniform submicron and nanoscale particles by laser pyrolysis. Highly uniform particles are desirable for the fabrication of a variety of devices including, for example, batteries, polishing compositions, catalysts, and phosphors for optical displays. Laser pyrolysis involves an intense light beam that drives the chemical reaction of a reactant stream to form highly uniform particles following the rapid quench of the stream after leaving the laser beam.